Accession Number : ADA113012

Title :   Identification of Flaws Responsible for Crack Initiation and Micromechanisms of Slow Crack Growth in the Delayed Fracture of Alumina.

Descriptive Note : Final rept. 1 Aug 77-31 Dec 81,

Corporate Author : CALIFORNIA UNIV LOS ANGELES DEPT OF MATERIALS SCIENCE AND ENGINEERING

Personal Author(s) : Sines,George ; Okada,Tatsuo

PDF Url : ADA113012

Report Date : Feb 1982

Pagination or Media Count : 232

Abstract : An eccentricially loaded column testing system was developed to produce a number of crack initiations on the surface of an alumina specimen. Using a specially developed fluorescent dye penetration technique, direct observations of early stages of crack extension from inherent flaws were made in the testing system. It was found that a larger number of inherent flaws exist on the surface of a ceramic and can, by themselves or as assemblies, become potential crack origins. Cracking sites were found to be multiple in delayed fracture. The initial stage of delayed fracture was shown to involve the interaction and coalescence of near-by flaws with extensive intergranular cracking. A model for the delayed fracture, the microscopic slow crack growth and crack interaction and coalescence was proposed. The model gives much better prediction of fracture times than does the conventional single-worst-crack approach. It was found that an assembly of microscopic cracks, which may not be observable, can be much more dangerous than a single macroscopic crack in delayed fracture and that the coalescence of the microscopic cracks may occur in a very short time without being identified because a critical amount of slow crack growth from the individual inherent flaws can be very small. (Author)

Descriptors :   *ALUMINUM OXIDES, *FRACTURE(MECHANICS), *CRACK PROPAGATION, *MICROCRACKING, *DEFECTS(MATERIALS), MECHANICAL PROPERTIES, LOADS(FORCES), MICROSTRUCTURE, PHASE TRANSFORMATIONS, SINGLE CRYSTALS, COALESCENCE, OPTICAL SCANNING, CREEP STRENGTH, STABILITY, STRESS CORROSION, MODULUS OF ELASTICITY, PHYSICAL PROPERTIES, CERAMIC MATERIALS

Subject Categories : Laminates and Composite Materials
      Properties of Metals and Alloys
      Mechanics

Distribution Statement : APPROVED FOR PUBLIC RELEASE